A transparent plastic disc is described in the Copending Application Ser. No. 627,701 now U.S. Pat. No. 3,430,966, issued Mar. 4, 1969 in which picture information in the form of video signals is recorded on one or both side of the disc. The recorded picture information on the disc is intended to be reproduced, for example, through a television receiver, by playing the disc on a turntable and by directing a light beam through the disc, as described in the Copending Application Ser. No. 507,474 now abandoned. The light beam is modulated by the video recordings on the disc, and a pick-up head is provided which responds to the resulting light signals to transform them into corresponding electrical video or picture signals for playback purposes.
The present invention is concerned with such a video disc record, and with a duplication process by which a multiplicity of such records may be mass-produced from a master record die. The process to be described, for example, is one in which the video information is embossed into one or both surfaces of the video disc record, rather than being stamped or molded into the surface. The material of the disc record surface is made such to be appropriate for embossing and to enable, under suitable temperature conditions, a slight force pressing the disc surface against a master die to cause the impressions on the surface of the die to be embossed into the surface of the disc. With such an embossing process, there is no transverse flow of the disc material, as occurs in the usual prior art stamping or molding processes, as are presently being used in the production of phonograph sound records, for example, and by which the actual surface of the record is raised above its melting point.
The stamping techniques presently being used in the manufacture of phonograph records are not suitable for the extraordinarily fine micro-grooves and patterns required by video frequency recordings of picture information. Such stamping techniques as are presently being used in the production of phonograph sound records require that the master record die be heated to a temperature above the melting point of the vinyl or other plastic material used in the phonograph record.
In the prior art phonograph record duplicating process, a "biscuit" of the vinyl or other plastic material is placed in a "stamper", and the heated master record die is brought down onto one or both surfaces of the biscuit. The plastic of the biscuit surface is melted and caused to flow radially into the spaces defined by the impressions on the master die surface. As mentioned above, this stamping technique by present day standards appears to be unsuited for the extremely fine micro-spiral grooves required for video frequency recordings.
As an alternative to the present day practice, and as will be described, a video disc record blank of laminated transparent plastic construction may be provided, the laminated record having a surface layer of relatively soft transparent plastic of any suitable known type, and which can be readily embossed; and a supporting base of a rigid plastic, such as an acrylic resin or polyvinyl chloride. As a first step in the alternate approach, the laminated disc record blank is heated to a point at which the surface tension of the surface material causes the surface to be smooth and regular. This temperature is the critical temperature at which embossed impressions may be formed on the disc surface, and it is below the melting point of the surface material.
The embossing die(s) is (are) heated to a temperature slightly above the critical temperature, and it (they) and the record blank are brought together with a slight pressure. As the die(s) and the record blank are brought together, the die(s) is (are) cooled to the aforesaid critical temperature, and its (their) surface impressions are embossed into the surface(s) of the record. Obviously, if two "sides" are being embossed, two embossing dies are required. The supporting structure would require modification, but such modification is well within the skill of the art.
After the disc record has been embossed, as described above, an opaque mask is deposited into the portions of its surface around the resulting embossed micro-grooves. This latter mask may be formed on the disc by using a vacuum deposition technique, as will be described.
The aforesaid disc record, when laminated in accordance with the aforesaid alternate approach, is used in order to present the desired surface characteristics for optimum embossing capabilities, and yet so that the record itself may be rugged and suitable for rough usage. The laminated structure of the record comprises reasonably tough and dimensionally stable clear plastic for the main body of the disc; and a plastic material on one or both surfaces of the disc which is most suited for embossing. The combination provides a video record disc which is useful, which can take an appropriate amount of handling, and which still can be embossed easily and effectively.